I love to run - especially trails. Absolutely the best high you can get. That said, I am biking now to save my knees and can't run over 5 miles without having pain for a week. 18 is no longer an option. Had you asked me when I was doing my regular 15-18 mile Saturday morning runs 10 years ago it would have been a no-brainer - the run, hands down. Almost any runner would say the same thing, and in fact, if I could get back to regular running I would give up biking in a heartbeat.